rsi: fix memory leak on failed URB submission

commit 47768297 upstream.

Make sure to free the skb on failed receive-URB submission (e.g. on
disconnect or currently also due to a missing endpoint).

Fixes: a1854fae ("rsi: improve RX packet handling in USB interface")
Cc: stable <stable@vger.kernel.org>     # 4.17
Cc: Prameela Rani Garnepudi <prameela.j04cs@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>